1. Field of the Invention
This invention relates to advanced ceramics, and in particular to a process utilizing fluidized bed chlorination to extract rare earth values from ore and the preparation of alkoxide precursors for the fabrication of such ceramics.
2. Description of Related Art
Doping of zirconia to provide a stabilized ceramic (either partially or fully) is well known, and generally required to avoid dimension problems due to phase changes with temperature, as pure zirconia is monoclinic below 1000.degree. C. and cubic above. Stabilization to the cubic phase has been provided by various oxides, particularly of 6-12% of rare earth, (especially yttrium) or by a somewhat larger percentage of calcium or magnesium. Rare earth stabilization to a tetragonal phase is also known. In an article entitled "Mechanical Properties and Thermal Stability of CeO Containing Tetragonal Zirconia Polycrystals" in the American Ceramic Society Bulletin (Vol. 65, No. 10, 1986, pp. 1386-1389) Tsukuma notes that about 10-16% cerium oxide, possibly with a small amount of Lanthanum and neodymium oxide, provided better stabilization to the tetragonal phase than yttrium oxide.
Generally ceramics are prepared from pressed powder, typically from mixed powders of the individual oxides (e.g. a mix of zirconium oxide and yttrium oxide powders) with the mix being milled and fired to provide a sintered ceramic. While alkoxide mixes have been experimentally cured and calcined to produce ceramics, difficulties have been incured due to trapped curing by-products and further, such a process has generally been considered too expensive for commercial production. Precipitation from a chloride solution followed by calcination has also been used to produce a powder for pressing, and a much more homogeneous product is produced.
Naitou et al. in U.S. Pat. No. 4,650,652, issued Mar. 7, 1987, relates to a process for recovering high purity rare earth oxides from a waste rare earth phosphor. The process utilizes dissolving waste rare earth phosphor in an excess amount of acid, adding oxalic acid to obtain precipitates of rare earth oxylates, washing precipitates and baking precipitates.
Ozaki et al, in U.S. Pat. No. 4,507,254, issued Mar. 26, 1985, relates production of a rare earth metal alkoxide by reacting a rare earth metal carboxylate with an alkali metal alkoxide in an inert organic solvent to liquid under anhydrous conditions.
U.S. Pat. No. 4,244,935, issued to Dell on Jan. 13, 1981, relates a method of forming the chloride of a metal-oxygen containing substance based on a fluid coking technique. It should be noted that the commercial process for making zirconium metal utilizes a fluidized bed process in which the ore is subjected to a chlorination step which produces a relatively impure, hafnium-containing zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easily separated by differential condensation). U.S. Pat. No. 3,895,097, issued to Langenhoff et al. on July 15, 1975, also relates to a process for reacting metal oxides with chlorine.
U.S. Pat. No. 4,670,573, issued to Greco et al. on June 2, 1987, relates to the preparation of metal alkoxides from metals and alcohols. The description of the prior art lists references that relate to the reaction of metals with alcohol to form metal alkoxides. Kirk-Othmer also discusses metal oxides of higher, unsaturated, or branched alcohols made from lower metal alkoxides on page 1, lines 25-50.
U.S. Pat. No. 4,472,510, issued to January on Sept. 18, 1984, relates to a process of making glassy ceramics, including a method of preparing a carbon-containing monolithic glassy ceramic including a metal alkoxide which hydrolyzes and polymerizes in the presence of water.